This invention relates generally to a multiplexing system adapted to sequentially convey data samples derived from a plurality of analog sensors to a receiving terminal, and more particularly to a protective arrangement for detecting the occurrence of an abnormal condition in any one of the sensor circuits and to indicate this condition.
In industrial process control, it is necessary to transmit data obtained at various points in the field to a central computer or control station. The data to be conveyed to the common receiving terminal may be changes in pressure, temperature, flow rate or any other process variable. Generally, this data is derived by means of individual analog sensors which convert the process variable at the various points into corresponding analog signals.
A telemetering system in which the output of each analog sensor is fed to the remote terminal over a separate wire line is usually not feasible, particularly when many sensors are involved. The large number of lines then entailed and their lengths make a multi-line system prohibitively expensive.
Multiplexing techniques are known which act to sequentially transmit digital values derived from data generated by analog sensors to a digital computer or other receiving terminal over a single main channel, thereby obviating the need for as many telemetering lines as there are sensors. A time-division multiplexing system of this type employs an electronic or mechanical commutator at the transmission station to sequentially sample the data produced by each analog sensor, the output of the commutator being applied to an analog-to-digital (A/D) converter.
To effect process control, the output of the A/D converter is applied to a central digital computer which functions by means of a receiver commutator running in synchronism with the transmitter commutator to sequentially control the sensed processes through final control elements. If, for example, each analog sensor in the analog sub-system is a thermocouple which senses the prevailing temperature in a process line, the final control element related to this sensor may be a valve adapted to supply a cooling medium to the line to an extent necessary to adjust the temperature therein to conform the process temperature to a set point with which the process variable is compared.
But if an abnormal condition develops in a given sensor, such as an open circuit, the operation of the associated final control element is then out of control, and in an unmonitored process this may have serious consequences. It is desirable, therefore, to provide a protective arrangement in conjunction with each analog sensor included in the multiplexing system to detect the occurrence of an abnormal condition and to sound an alarm or afford some other indication so that an operator can be alerted to take steps to prevent damage to the unmonitored equipment or process.
One known type of protective arrangement for this purpose takes the form of a centrally-powered detector which applies an offset current to the common line extending between the commutator coupled to the plurality of analog sensors and the A/D converter that sequentially converts the sampled analog values derived from these sensors into the corresponding digital signals.
With this known protective arrangement, if the particular analog sensor being sampled by the commutator is in its normal low-impedance closed state, the A/D converter will then convert the analog value produced thereby into a digital value lying within a valid range. But if the sampled analog sensor circuit is open and therefore defective, then the offset current will generate an exceptionally large offset voltage across the open impedance, and the A/D converter will then convert this voltage to a digital value well beyond the valid range. The indication resulting from this abnormal value provides the required alarm.
A centrally-powered protective arrangement which produces an offset current common to all of the sequentially-sampled analog sensor circuits introduces an error value which is added to each analog input and creates a problem that dictates some means to effect error correction. But a more serious drawback of this known arrangement is that it precludes the use of a noise rejection R-C filter in conjunction with each analog sensor circuit.
A filter of this type serves to discriminate between the useful analog signal and background noise transients, thereby enhancing the signal-to-noise ratio of the analog input sub-system. However, with a centrally-powered protective arrangement, input filtering cannot be used, for the filter capacitor looks like a short circuit to the offset current pulse. This limits the normal-mode noise rejection ability of the entire analog input sub-system.
To overcome the deficiencies of a centrally-powered protective arrangement and make it possible to include a noise rejection filter for each sensor, it is known to provide an individually-powered arrangement wherein offset current is applied to each analog sensor input line from an individual power source in series with a resistor of high value. Inasmuch as a protective arrangement in accordance with the invention also applies an offset current to each sensor input line, the distinctions between the previously known protective arrangement of this type and the present arrangement and the advantages to be gained by the latter will explained in the subsequent section of this specification which describes the invention in detail.